1. Field of the Invention
This invention pertains to a portable life support system and, more particularly, an improved portable life support system employing a liquid cryogen to provide temperature regulation and breathable atmosphere for the wearer of a garment or suit.
2. Description of the Prior Art
Portable life support systems are typically used in environments that are uninhabitable or otherwise hostile to humans. Examples of such environments include space, underwater, fire fighting, and hazardous materials handling. The two most critical requirements of a portable life support system, from an environmental control perspective, are providing body temperature regulation and a breathable atmosphere for the users.
Technical requirements for such systems vary widely and are constrained by activity type and performance level. However, generally desirable characteristics of personal portable life support systems include a weight that can be carried by a single person, a size that can be carried by a single person without undue loss of mobility, operation without the necessity for an umbilical, and a design that will not impair the dexterity of the user. Although these characteristics may vary depending on the environment, they are generally desirable and generally consistent.
Subsystems providing breathable atmosphere for portable life support systems are generally classed as either open circuit, semi-closed circuit, or closed circuit, depending on the proportion of atmosphere recirculated. In an open circuit subsystem, atmosphere is immediately vented from the system upon exhalation by the user whereas all atmosphere exhaled is recycled in a closed system. A semi-closed system falls somewhere in between, venting significant amounts of exhaled atmosphere but also recirculating large amounts.
Breathable atmosphere subsystems may also be grouped according to the type of breathing mechanism employed. The simplest type is the "free flow" system, wherein atmosphere is provided to the user at a continuous, relatively constant rate regardless of the level of activity. A "demand" system employs a demand regulator like those used with SCUBA equipment to provide breathable atmosphere only when the user inhales atmosphere, i.e., on demand. Demand systems can be employed orally or with a combined oral and nasal delivery.
The need for a hazardous material suit for liquid fuel handlers prompted efforts by the United States Army in this area by the early 1960's. These efforts were primarily directed at systems having separate subsystems dedicated to either body temperature regulation or breathing, but were eventually directed to the use of "liquid air" as a cryogenic fluid to provide both body temperature regulation and breathable atmosphere. The design incorporated a straight semi-closed circuit breathing system in which a breathable atmosphere was generated by vaporizing liquid air. The vaporization process provided minimal cooling and body temperature regulation was achieved through circulation and recirculation of vaporized cryogen through the user's suit.
A cryogenic fluid may be defined as a fluid which boils (i.e., changes state from liquid to gas) at temperatures less than approximately 110K at atmospheric pressure. Examples of the cryogen include both nitrogen and oxygen (the primary components of "liquid air") as well as hydrogen, helium and methane. As used herein, "cryogen" shall refer to a cryogenic fluid and "cryogenic technology" shall refer to knowledge, techniques, and equipment for harnessing the physical properties of cryogenic fluids to practical applications.
However, cryogenic technology in portable life support systems quickly encountered many technical constraints. Portable life support system technology furthermore diverged from the approach in the early Army studies to create two schools of thought as the technology matured. One school of thought continued to use cryogen for cooling and to generate breathable atmosphere. This approach is disclosed in U.S. Pat. No. 3,064,448 issued to P. E. Whittington, U.S. Pat. No. 3,117,426 issued to R. A. Fischer et al., U.S. Pat. No. 3,227,208 issued to V. L. Potter, Jr. et al., and U.S. Pat. No. 2,990,695 issued to D. A. Leffingwell, Jr.
The divergent school of thought was prompted by efforts to achieve breakthroughs in efficiency and began by separating the body temperature regulation and breathing subsystems. Examples of this school of thought are found in U.S. Pat. No. 4,172,454 issued to Warnecke et al., and U.S. Pat. Nos. 4,286,439 and 4,459,822 issued to Pasternack. The separation of temperature regulation and breathing subsystems removed technical constraints to permit use of more effective and more dangerous coolants that were not cryogenic fluids. As noted in the '454 Warnecke et al. patent, some in the art switched to solid coolants such as dry ice instead of cryogenic fluids.
One of the primary difficulties that led to the divergence of thought was that the early applications of cryogenic technology could not produce high efficiencies in terms of adequate and controllable body cooling and duration per unit weight. We have discovered that the source of this problem was ineffective heat exchange. Cooling was primarily provided by heat exchange between circulating air and the user's body i.e., in a gas phase loop. Our invention takes advantage of the fact that heat exchange in a liquid phase loop is far superior to heat exchange in a gas phase loop. This superiority arises from a number of factors, foremost of which is greater control over the heat exchange process.
A second difficulty that still plagues cryogenic life support systems arises from the reliance of such systems on user orientation relative to the field of gravity. Breathable atmosphere subsystems employing air under pressure, such as SCUBA, are "orientationally independent" because the gas under pressure will expand through its natural properties to provide constant supply to the user. However, liquids perform fundamentally differently and require a motive force for delivery to the point of heat exchange where they vaporize. Virtually all cryogenic systems known heretofore employ gravity by storing the liquid cryogen relative to the point of heat exchange and current "orientationally independent" delivery systems are inefficient and costly.
The current systems deliver the dewar contents by separating the vaporized cryogen in the dewar, which is then pressurized, and the liquid cryogen, which is expelled by the force exerted by pressurized vaporized cryogen in the dewar. The separation results from the differing effects of gravity on the liquid cryogen and the vaporized cryogen and operates to separate them. An intake port in the dewar is submerged by the separated liquid cryogen which is then delivered by the further effects of gravity. Relying on gravity therefore causes a marked decrease in performance because whenever the system user affects the orientation of the delivery with respect to the gravitational field, the dewar contents lose pressurization and delivery becomes less effective as the vaporized cryogen is rapidly vented through the intake port which is no longer submerged in the liquid cryogen.
The problem is compounded in space where there is only negligible gravity regardless of orientation. Most systems in space therefore use "liquid acquisition devices" which employ an extremely fine mesh to separate the gas and liquid phases. However, the mesh is extremely fine and consequently very sensitive to manufacturing tolerances and very expensive. Also, liquid acquisition devices must be "tuned" to the particular cryogen in use and so are not readily adaptable to a wide variety of cryogens. There consequently also is some debate as to whether liquid acquisition devices are efficiently operable with cryogenic mixtures comprising two or more cryogens having separately identifiable physical properties.
Thus, the inability to achieve effective heat transfer and to develop a satisfactory orientationally independent delivery caused some in the art to abandon cryogenic technology or to adopt undesirable but necessary alternatives. These shortcomings affected both primary functions of the system and resulted in the functional division of the system as first proposed in the 1960's into separate, functionally dedicated, subsystems. This, in turn, led to the return of compressed gas for breathable atmospheres and dangerous liquid coolants, and even solid coolants, for temperature regulation.
It is therefore an object of this invention to overcome these problems with portable life support system employing cryogenic technology that effectively combines body temperature regulation and delivery of a breathable atmosphere.
It is furthermore an object of this invention that, in various embodiments, the system employs breathable atmosphere subsystems in open circuit, semi-closed circuit, and closed circuit configurations with either straight or demand supply via either oral or combination oral/nasal regulators.
It is a still further object of this invention to provide a system which employs orientationally independent delivery of liquid cryogen from storage to the point of heat exchange.